"Merlin" fans get double the Gaius this week when James Callis, who played Gaius Baltar on "Battlestar Galactica," guest stars as court physician Gaius' former pupil.

Callis deepens his voice and alters his accent to play devious Julius Borden in "Aithusa," which airs at 9 p.m. Jan. 27 on Syfy. Borden comes to Camelot looking for the third part of a Triskelion—an ancient object that supposedly will reveal the location of the last dragon egg.

Gaius (Richard Wilson) refuses to help Julius, but Merlin (Colin Morgan) does, believing that he will fulfill his duty as the last Dragonlord to protect the dragon species. But Merlin is quickly tricked by Julius, and when King Arthur (Bradley James) and his knights set off to find Julius and the destroy the egg, Merlin must balance his loyalty to Arthur with his obligation as a dragonlord.

The episode isn't as dark as earlier episodes this season, which has seen an increase in viewers since the season premiere. Last week, 1.8 million people tuned in to watch on Syfy, and the show remains a huge hit worldwide. That can be attributed to its mix of danger, action and humor, which is apparent in "Aithusa." Especially fun are the scenes with Arthur, Merlin and the Knights of the Roundtable as they hunt Julius and search for the egg.

"Aithusa" also sees the return of the great dragon Kilgharrah (voiced by John Hurt), who sends Merlin on his quest to save the dragon egg.

"There's been all these questions of trust between them," Morgan said in a conference call earlier this season. "But the dragon is a creature of magic as much as Merlin is. And there's that connection between them."

That connection deepens by the end of "Aithusa." (Watch four sneak peek clips of the episode in the player above.)